Examples:
"Sea King" is great, especially in the first half, where it focuses on him as a warrior.
"Across Eternity" has a lot of this "extreme competence" as a warrior.
"Aftermath" is pretty close.
"Greenies" war segments, of course.
I.e., competent warrior stories without too much power creep or extreme fantasy.
You might like the Clan Amir series by Ernest Bywater.
Perhaps also his Al Adams stories - Finding Home (2 versions) and the Chaos Calls series in the Damsels in Distress universe, starting with Learning Visit.
A number in the Damsels in Distress universe concentrate on the Hero as warrior.
Yippee, I can be first to suggest 'The Private'
by Random Writings. It's inactive and incomplete but it's one of those stories that are worth reading anyway.
AJ
https://storiesonline.net/universe/979/grim-reaper-universe
From childhood to retirement. MC starts off fighting school bullies and playing football as a kid, with no econonomic options he ends up in the army in Iraq, where he finds his calling as an avatar of death. patrols and ambushes, hostage rescues and downed choppers, artillery fire missions and USO concerts. He survives but with severe PTSD. Back home, now a SWAT cop and police trainer, he deals with drug mules, spree killers, terrorists and politics.
Quite a long series and well worth reading.
oh yes, i so second this. the whole Reaper series is just *chef's kiss*. i havent come across many like it.
The Earl's Man, Of Honor and Betrayal, The Goatherd, and Spirit Quest are all essentially 'Lone man (or boy) Overcomes and Makes a Difference' stories as well.
The Scout finished by fantasylover is another one that might interest you.
https://storiesonline.net/s/19671/scout-by-fantasylover
The Scot has two that might be worthwhile
https://storiesonline.net/a/the-scot
Gunslinger and Winds of Change
Todd_d172 has a whole bunch of related short stories under https://storiesonline.net/universe/921/tales-from-the-shack, in most cases the warriors have retired but are still extremely cantankerous if required.
He also wrote https://storiesonline.net/s/27763/luthers-wars which is related to https://storiesonline.net/universe/932/saint-clair (retired GI joins the police).
https://storiesonline.net/s/17722/mausfalle would also come close, stand-alone.
https://storiesonline.net/s/15444/the-song-of-farwalker-and-deadgirl - ditto.
Now turning to Argon, https://storiesonline.net/s/58363/joerg-isebrand is set in the 16th century.
The stories in https://storiesonline.net/universe/102/anthony-carter are mostly set in the British navy during the Napoleonic wars.
https://storiesonline.net/s/22134/the-healer-by-qm by QM (as it says) fits, also by QM are two of the stories in https://storiesonline.net/universe/934/original-realm-of-magic-and-magical-beings and those in https://storiesonline.net/series/1769/shadow-hunter (one is still being posted).
I believe both Todd and QM were in the military in some capacity.
Mike Cropo's Spartan Chronicles is a good but unfinished LONG series with I wouldn't call extreme fantasy but a decidedly science fiction bent but no more than some of the other suggestions. Mike was in the military so his stories are quite realistic.
Ka Hamd also has a bunch of warrior type stories. He is a veteran so there is an authentic feel to the stories.
Clar War series
Tour of Duty
A life in the Service
Ori War
Just to name a few of the longer ones.